1. Technical Field
The present invention relates to a technique of controlling a light-emitting device such as an organic light-emitting device (hereinafter, referred to as ‘OLED’).
2. Related Art
Light-emitting devices for controlling light-emitting elements to desired brightness levels depending on pulse widths or current values of signals (hereinafter, referred to as ‘driving signals’ provided to the light-emitting elements have been proposed. In such a light-emitting device, a brightness variation caused by errors (deviations) in the characteristics of individual light-emitting elements becomes a problem. Techniques for suppressing such a brightness variation by correcting current values of driving signals depending on real characteristics of light-emitting elements are disclosed in, for example, JP-A-2005-103914 and JP-A-2005-81696. Also, techniques for suppressing such a brightness variation by correcting pulse widths of driving signals depending on the errors in the characteristics of light-emitting elements are disclosed in JP-A-2005-103914 and JP-A-2005-103816.
However, an aspect of variation in the characteristic of each light-emitting element over time (for example, a characteristic degrading speed) varies depending on a correct amount (variation caused by correction) in the pulse width or the current value of the driving signal. Therefore, even though the luminous energies of the individual light-emitting elements is temporarily unified by the correction of the pulse width or the current value as in the techniques exemplified above, the deviation in the characteristics of the individual light-emitting elements increases over time. This point will be described below in detail.
FIG. 8 is a graph showing the relationship between the real light amount (vertical axis) of each light-emitting element when the same grayscale value is designated for two light-emitting elements A and B (having characteristics Fa and Fb, respectively), and accumulated time for which the corresponding light-emitting element has been used. In FIG. 8, it is assumed that the light amount of the light-emitting element A and the light amount of the light-emitting element B are different from each other by ‘ΔP’ at a time point t0 (in an initial stage) due to the error between the characteristics of the light-emitting elements A and B. According to the techniques disclosed in JP-A-2005-03914, JP-A-2005-81696, and JP-A-2005-103816, it is possible to make the light amounts of the light-emitting elements A and B almost equal to each other by, for example, increasing the pulse widths or the current values of the driving signals supplied to the light-emitting elements.
However, the aspect of the variation in the light amount of the light-emitting element B over time is changed from the characteristic Fb to a characteristic Fb1 by the correction of the pulse width or the current value. As understood from the characteristic Fb1, the speed at which the light amount of the light-emitting element B decreases over time (hereinafter, referred to as ‘a deteriorating speed’) is higher than the deteriorating speed (characteristic Fb) of the light-emitting element B before the correction or the deteriorating speed (characteristic Fa) of the light-emitting element A due to the increase in the pulse width or the current value of the driving signal. Therefore, the difference between the light amounts of the light-emitting elements A and B increases over time as compared to a case in which the driving signal is not corrected. For example, even though the light emission amounts of the light-emitting elements A and B are unified at the time point t0, the difference ΔP1 between the light emission amounts of the light-emitting elements at a time point t1 is larger as compared to the case in which the driving signal is not corrected.